1. Field of the Invention
The present invention relates to a lithium-ion rechargeable battery charging method, and in particular to a charging method that detects a temperature of a battery and controls a charging current based on the detected temperature. In this specification, a “lithium-ion rechargeable battery” is used in a sense that includes a lithium-polymer battery.
2. Description of the Related Art
In order to improve safety, a charging current and a charging voltage of lithium-ion rechargeable batteries are controlled according to a temperature of the lithium-ion rechargeable batteries. FIG. 1 shows an exemplary relationship of a charging current and a maximum voltage to a temperature of the lithium-ion rechargeable battery. As shown in this figure, a voltage and a current of the lithium-ion rechargeable battery are set high in a normal temperature range, but both charging voltage and current of lithium-ion rechargeable battery are set low in high and low temperature ranges. For this reason, if a temperature of a battery rises to the high temperature range in a charging operation, a charging voltage is reduced, and as a result the battery cannot be fully charged. In a conventional constant-current and constant-voltage charging operation of a lithium ion battery, in particular, at the start of a charging operation, in the case where a temperature of the battery is high, for example, as shown in FIG. 1, if a battery temperature rises from the normal temperature range to the high temperature range, charging voltage and current in the high temperature range are lower than in the normal temperature range. For this reason, there are problems in that the completed charged amount of the battery is low, or charging operation time is long. In the case of a charging circuit that cannot reduce a charging voltage, there is a problem in that the charging circuit stops the charging operation and as a result the completed charged amount of the battery is low.
A method has been developed which prevents the temperature of a charged battery from rising too high. This method is disclosed in Japanese Laid-Open Patent Publication No. H07-73907-A (1995). In this method, a temperature of a nickel-hydrogen battery is held in a range of 10° C. to 40° C. in a charging operation. In addition to this, in this charging method, a battery temperature rise value ΔT is measured, and a battery temperature at charging operation completion is predicted based on a battery temperature T and a value ΔT/Δt. If it is determined that the predicted battery temperature will exceed 40° C., the battery is cooled.
Also, a method has been developed which detects a temperature of a battery and controls a charging current. This method is disclosed in Japanese Laid-Open Patent Publication No. H07-222374-A (1995). This method also charges a nickel-hydrogen battery. A temperature rise gradient is detected, and a temperature is predicted based on the detected temperature rise gradient. Accordingly, a charging Current is controlled based on the predicted temperature.